Field Emission Gun Scanning Transmission Electron Microscope Research Articles

Quantitative Thin-Film X-ray Microanalysis by STEM/HAADF: Statistical Analysis for Precision and Accuracy Determination

Silicon-germanium thin films have been analyzed by EDS microanalysis in a field emission gun scanning transmission electron microscope (FEG-STEM) equipped with a high angular dark-field detector (STEM/HAADF). Several spectra have been acquired in the same homogeneous area of the cross-sectioned sample by drift-corrected linescan acquisitions. The Ge concentrations and the local film thickness have been obtained by using a previously described Monte Carlo based "two tilt angles" method. Although the concentrations are in excellent agreement with the known values, the resulting confidence intervals are not as good as expected from the precision in beam positioning and tilt angle position and readout offered by our state-of-the-art microscope. The Gaussian shape of the SiKalpha and GeKalpha X-ray intensities allows one to use the parametric bootstrap method of statistics, whereby it becomes possible to perform the same quantitative analysis in sample regions of different compositions and thicknesses, but by doing only one measurement at the two angles.

Quantitative characterization of nanoprecipitates in irradiated low-alloy steels: advances in the application of FEG-STEM quantitative microanalysis to real materials

The characterization of the solute-enriched features (clusters or nanoprecipitates in irradiated low-alloy steels) requires extremely high spatial and elemental resolution, previously necessitating analysis using atom probe field-ion microscopy. In this investigation, field-emission gun-scanning transmission electron microscope (FEG-STEM) quantitative energy dispersive X-ray (EDX) microanalysis (spectrum imaging) has been applied to the characterization of the irradiation-induced nanoprecipitates in a low-alloy forging steel. Refinements in the EDX data have been possible via the application of multivariate statistical analysis (MSA) to the spectrum images, resulting in significantly reduced noise in the images. Most importantly, MSA permitted the clear identification of other elements in these Ni-enriched nanoprecipitates—including Mn and Cu. The processed X-ray spectrum images also provided direct evidence of the preferential formation of these irradiation-induced features along pre-existing dislocations within the steel, as well as the formation of intragranular nanoprecipitates. This research has provided the first direct X-ray spectrum images of irradiation-induced nanoprecipitates in high Ni A508 Gr4N forging steel, and has demonstrated the significant improvements attainable though the application of MSA techniques to the spectrum images. These results independently confirmed the analyses of the Ni-enriched nanoprecipitates previously conducted by 3D-APFIM, with the performance of the FEG-STEM/EDX technique shown to be comparable to that of the 3D-APFIM technique.

Grain boundary segregation of phosphorus and molybdenum in A533B steel

Phosphorus and molybdenum segregation to grain boundaries in a commercial grade A533B steel subjected to a variety of heat treatments has been examined using a field emission gun scanning transmission electron microscope (FEGSTEM) with energy dispersive X-ray micro-analysis. The results indicate that P and Mo concentrations at prior austenite grain boundaries increase with aging time. This follows the prediction of McLean's equilibrium segregation model, when modified to take account of the interaction energy between phosphorus and molybdenum.

Effect of phosphorus segregation on fracture properties of two quenched and tempered structural steels

Segregation of phosphorus to grain boundaries in commercial grade 2·25Cr1Mo and A533B steels subjected to a variety of heat treatments has been examined using field emission gun scanning transmission electron microscope (FEGSTEM) with energy dispersive X-ray micro-analysis. The measured grain boundary P concentrations have been used to explain the low temperature brittle fracture behaviour, which exhibits increasing levels of intergranular embrittlement as a function of aging time at 520°C. The results show that P segregation at grain boundaries has a strong effect on the intergranular area fraction, the microscopic fracture stress, and the fracture toughness. Phosphorus peak height ratios measured using auger electron spectroscopy are shown to follow a linear relationship with the FEGSTEM/X-ray values.

Kinetics of phosphorus segregation and its effect on low temperature fracture behaviour in 2.25Cr–1Mo pressure vessel steel

Low alloy steels operating for long times at temperatures of ∼ 500°C may be sensitive to embrittlement owing to the segregation of various trace elements to prior austenite grain boundaries and/or carbide/matrix interfaces. This type of segregation can adversely affect the resistance of the steel to fracture; this is associated with a change in fracture mode from transgranular to intergranular. The present paper describes research carried out to examine the effects of aging at 520°C following 'full' tempering at 650°C on the kinetics of phosphorus segregation in 2.25Cr–1Mo steel, and any subsequent effects on fracture resistance and fracture mode. Fracture toughness tests have been conducted at low temperatures and the fracture surfaces have been examined using scanning electron microscopy (SEM) with EDX. Auger electron spectroscopy (AES) and some limited microanalysis in a field emission gun scanning transmission electron microscope (FEGSTEM) have been employed to measure phosphorus segregation at grain boundaries It is observed that full tempering changes the kinetics of phosphorus segregation during aging from that associated with a steel aged directly at 520°C after quenching. This observation is ascribed to the formation of molybdenum rich alloy carbide precipitates at 650°C, which enables phosphorus atoms to be released from Mo–P clusters. Subsequent aging at 520°C allows the released phosphorus to segregate to grain boundaries and carbide/matrix interfaces. No segregation induced embrittlement is observed in the quenched steel, when aged directly at 520°C for a comparable period of time.

On the Effects of Spherical Aberration and Aperture Misalignment on the Formation of Small Electron Probes

A major reason for performing microanalysis in the Field-Emission Gun Scanning Transmission Electron Microscope (FEG-STEM) is the very high spatial resolution of the information obtained. It has been estimated that in ideal cases, energy-dispersive x-ray analysis in such an instrument can provide 0.1 wt.% detection sensitivity for an element in a region about 1.5nm in diameter of a foil about 30nm thick. It is obvious that such performance requires that the instrument be properly adjusted, and hence that the probe-forming characteristics be fully understood.There are three fundamental limits on the minimum size of an electron probe, these being i) the geometrical demagnification of the source, ii) diffraction at the beam-limiting aperture, and iii) spherical aberration in the probe-forming lens. In addition, misalignment of the beam-limiting aperture will result in probe aberrations.

Grain boundary segregation in an oxide-dispersion-strengthened ferritic steel

Oxide-dispersion-strengthened (ODS) ferritic steels are being developed in the nuclear industry for advanced fast and fusion reactor systems [1, 2]. These materials offer improved elevatedtemperature strength characteristics compared with the currently favoured 12% Cr martensitic alloys [3], together with high swelling resistance under irradiation [4]. Attention is presently focused on the MA 957 superalloy [nominal composition Fe-14Cr1Ti-0.3Mo-0.27Y203 (wt%)], which contains a fine yttria dispersion in a fully ferritic 14% Cr matrix, and is manufactured by the powder metallurgy route of mechanical alloying, viz. ball-milling, compacting and extrusion [5]. Significant enhancement of creep ductility is possible in ODS materials for matrix grain structures of high aspect ratio [6]. Such directional grain coarsening is conventionally achieved by a process of zone annealling, which essentially results in secondary recrystallization [7]. The secondary recrystallization response of extruded bars of MA 957 has recently been characterized by Evens et al. [8]. In these studies a significant curvature of the recrystallization interface was noted in longitudinal sections of partially zone-annealled samples, the interface becoming more steeply inclined to the normal with increasing zoning rate. The curvature of the interface is shown in Fig. la and is not thought to arise from variations in temperature across the section of the bar. This is confirmed by holding the specimen stationary in the temperature gradient for 30 min; this would result in a planar interface if a diametral temperature gradient exists, but the interface remains curved, as shown in Fig. lb. A similar curved interface has been observed by Marsh and Martin [9] in MA 6000 (a yttriastrengthened nickel-based superalloy), which was ascribed to strain gradients in the original extrusion, leading to differences in texture and hence grain boundary mobility. Evens et al. [8] alternatively suggest that in MA 957 ferritic superalloy the phenomenon is a consequence of gradients in solute segregation which influence grain boundary mobility. Therefore, in order to provide more information on this topic, an accurate analysis of grain boundary microchemistry was carried out on MA 957 using a field-emission gun scanning transmission electron microscope (FEGSTEM).

Performance of the Gatan PEELS™ on the VG HB501 STEM

Parallel-detection electron energy-loss spectrometers offer several hundred times the detection efficiency of serial-detection spectrometers, as well as improved energy resolution. These advantages should be especially important when using a scanning transmission electron microscope (STEM) with a cold field emission gun (FEG), in which the available beam current is typically 10 to 100 times less than in a conventional TEM, while the beam energy spread is typically only 0.3 eV. We have therefore investigated the performance of the Gatan parallel-detection spectrometer (Gatan model 666 PEELS™) when mounted on the VG HB501 FEG STEM.